Numerical study on the in-plane behaviour of existing timber diaphragms strengthened with diagonal sheathing

The vulnerability assessment of heritage buildings in earthquake prone areas is greatly dependent on the quality of the diaphragms. Inefficient wall-to-diaphragm connection and/or insufficient in-plane strength and stiffness of the diaphragms often lead to extremely ruinous failures characterized by of entire building-portions losing their stability (I mode collapses). In traditional buildings, the diaphragms are mostly comprised of flexible wood diaphragms where a layer of floorboards is supported on regularly spaced timber joists spanning perpendicularly to the board direction. The nail couples that fix the floorboards to the underlying framing, determine the in-plane response of such diaphragms which consequently can be inadequate to preventing the out-of-plane collapse of the face-loaded walls in case of strong earthquakes. The application of an additional layer of timber planks that are nailed to the original flooring at a 45° inclination to the joists, can significantly improve the in-plane behaviour of the diaphragm by creating strut-and-tie resisting mechanism that engage the timber elements (truss analogy). This retrofit solution is cost-effective and it is also welcomed by the heritage agencies because of its aesthetic consistency with the original diaphragm condition. The outcomes of a parametric study mainly based on nonlinear static analyses is presented herein. Among the various aspects included in the study, there are: aspect ratio, scale effect, geometrical and mechanical properties of the sub-components. Two different numerical approaches were considered and validated on experimental data available in literature. Nonlinear dynamic analyses were also conducted in order to evaluate the diaphragm energy dissipation and displacement demand.